Since the recent losses of several atmospheric instruments with good vertical sampling capabilities (SAGE II, SAGE III, GOMOS, SCIAMACHY, MIPAS), the scientific community is left with very few sounders delivering concentration profiles of key atmospheric species for understanding atmospheric processes and monitoring the Earth's radiative balance. The situation is so critical that, less than five such instruments will be on duty (most probably only 2 or 3) at the horizon 2020, whereas their number topped more than 15 in the years 2000. In parallel, recent inter-comparison exercises among the climate chemistry models (CCM) and instrument datasets have shown large differences in vertical distribution of constituents (SPARC CCMVal and Data Initiative), stressing the need for more accurate vertically-resolved data at all latitudes. In this frame, the Belgian Institute for Space Aeronomy (IASB-BIRA) proposed a small mission called ALTIUS (Atmospheric Limb Tracker for the Investigation of the Upcoming Stratosphere), which is currently in preliminary design phase (phase B according to ESA standards). Taking advantage of the good performances of the PROBA platform (PRoject for On-Board Autonomy) in terms of pointing precision and accuracy, onboard processing ressources, and agility, the ALTIUS concept relies on a hyperspectral imager observing limbscattered radiance and solar/stellar occultations every orbit. The objective is twofold: compared to scanning instruments, the imaging feature allows to better assess the tangent height of the sounded air masses (through easier star tracker information validation by recognition of scene details), while its spectral capabilities will be good enough to exploit the characteristic signatures of many molecular absorption cross-sections (O3, NO2, CH4, H2O, aerosols,⋯). The payload will be divided in three independent optical channels, associated to separated spectral ranges (UV: 250-450 nm, VIS: 440-800 nm, NIR: 900-1800 nm). This approach also offers better risk mitigation in case of failure of one channel. In each channel, the spectral filter will be an acousto-optical tunable filter (AOTF). Such devices offer reasonable étendue with good spectral resolution and excellent robustness and compactness and TeO2-based AOTF's have already been used in space missions towards Mars and Venus (MEX and VEX, ESA). While such TeO2 crystals are common in VIS-NIR applications, they are not transparent below 350 nm. Recent progresses towards UV AOTF's have been made with the advent of KH2PO4-based (KDP) filters. Through collaboration with the Moscow State University (MSU), several experiments were conducted on a KDP AOTF and gave confidence on this material. Here, we present the general concept of ALTIUS and its optical design with particular attention on the AOTF. Several results obtained with optical breadboards for the UV and VIS ranges will be exposed, such as the O3 and NO2 absorption cross-section measurements, or spectral images. These results illustrate the spectral and optical performances to be expected from an AOTF-based hyperspectral imager. Their implications for the ALTIUS mission will be discussed.@en

The performance of an acousto-optical tunable filter (AOTF) depends on several key parameters, such as the diffraction efficiency and the applied RF signal at the external network of the transducer. The latter has a specific bandwidth in which its efficiency is high. The goal of this paper is to experimentally investigate the relationship between the electrically matched RF bandwidth and the achieved diffraction efficiency, especially at the edges of the RF frequency band. The experiment revealed that the impedance matching is not completely correlated to the diffraction efficiency. Temperature measurements on the AOTF under test show that, at one edge of the tuning range, the RF signal is converted into a noninteracting acoustic mode.@en

Spectral imaging is a powerful tool for remote sensing applications. Among other valuable options, acousto-optical tunable filters (AOTF) offer interesting properties to this class of instruments. When sensitivity to ultra-violet (UV) light is required, KH2PO4 crystal (KDP) is a good choice for the acousto-optic (AO) interaction medium as its AO figure of merit is relatively large compared to other materials transparent in UV. However, it generally requires more acoustic power than well-known TeO2-based devices to achieve the highest diffraction efficiency. We report on measurements performed with a KDP-based AOTF (9-cut) driven with RF signals up to 3 W. It has been found that owing to various effects (Joules, acoustic wave attenuation,...) the heating of the crystal by a few degrees yields severe alteration of the AOTF spectral transmission function (STF): shift of the central wavelength, increase of the bandwidth and decrease of the peak filter efficiency have been observed. These three effects degrade the AOTF overall performance if not taken into account. This paper presents the experimental evidences and shows how a temperature-dependent AOTF model allows to derive the inner temperature of the device. Possible reasons for the flattening of the STF are also discussed.@en

The ALTIUS-instrument is a three-channel spectral imager, measuring in the ultraviolet, visible and near infrared wavelength domains, that is bound to fly aboard a PROBA-satellite. The goal of the ALTIUS-instrument is to make hyper spectral images of the limb of the earth. In each of ALTIUS' three channels an AOTF (Acousto-Optical Tunable Filter) will be used. For each channel an RF-generator will be developed and subjected to a suite of environmental tests such as thermal-vacuum, vibration, radiation, shock and Electromagnetic Compatibility (EMC). For the AOTF RF-generator different solutions are possible. Currently the design of an analog Hilbert transform and a high frequency phase-locked loop (PLL) are proposed as possible solutions. The choice of components, the PCB-design and the manufacturing of both designs has to be done in accordance to space qualified standards. This limits the choice of design and lifts the design challenge to a higher level. Because of the limited choice of space qualified components, general commercial approaches are not possible. For this reason specific designs have to be investigated and research has to be done to select the appropriate solution.@en

we examined acousto-optic interaction in the KH2PO4 crystal used in devices providing control of optical radiation parameters. The crystal was applied in a wide-angle tunable acousto-optic filter capable of image processing in the ultraviolet region of spectrum. The filter operated at different temperature conditions because of changes of environmental temperature and also due to heating of the crystal by driving electric and acoustic power. As found experimentally and proved theoretically, changes in the temperature of the instrument and appearance of thermal gradients in the crystal volume sufficiently influenced on operation characteristics of the acousto-optic device.@en

Wide aperture acousto-optical tunable filters (AOTF) appeared in the 70’s and found applications in various areas such as agriculture (crop stress monitoring), food industry (product quality), biology (fluorescence spectroscopy), etc. Their main advantages are the robustness, compactness, low power consumption, high filtering efficiency, tunability, potentially high spectral resolution (<1nm) and good image quality. In general, they offer interesting features to meet the needs in hyperspectral imaging applications. The physical process behind the wide aperture AOTF is the interaction of light and sound inside a birefringent crystal. For a given acoustic frequency, only photons of particular energy (i.e. wavelength) will couple with the acoustic wave, then leave the crystal in a slightly different direction than the rest of the light. By focusing the diverted beam onto a detector, one effectively performs a spectral image of the scene. Selecting another window of the light spectrum only requires the tuning of the sound frequency. A piezoelectric transducer bonded to the crystal is responsible for converting the electrical RF-signal into an acoustic wave. As most AOTFs operating in the visible and near-infrared domains are driven with frequencies ranging a few MHz to several hundreds of MHz, they do not necessitate particular electronics equipment. This statement does not hold when it comes to operating AOTFs in space environment: the RF-driving chain must be made from the limited catalogue of space-qualified parts. The problem gets even bigger when frequencies of several tens of MHz or higher must be generated. The work reported in this paper addresses different solutions to the problem of building a space-qualified RF-chain for an AOTF. This research was undertaken as part of the development of the ALTIUS space mission (atmospheric limb tracker for the investigation of the upcoming stratosphere) which aims at the measurement of atmospheric trace species (ozone, nitrogen dioxide, methane, water vapor,…) concentration profiles with a high spatial resolution. The measurement concept relies on the acquisition of spectral images of the bright atmospheric limb at well-chosen wavelengths. The imager concept allows to discope the need for scanning the atmosphere, as it was done by previous remote sensing missions. The instrument will be mounted onboard a PROBA-satellite (Project for On-Board Autonomy). The PROBA-satellite is a platform containing all the essential subsystems like a GPS, an attitude and orbit control system etc. to facilitate the payload which is placed on top of the platform. The complete project (platform and payload) is developed under the supervision of ESA (European Space Agency) and with funding of the Belgian Science Policy Office (BELSPO). The original ALTIUS concept made use of three independent spectral imagers (channels), each of them relying on an AOTF capable of isolating narrow passbands across the channel spectral range (ultraviolet (UV): from 250 to 400nm, Visible: from 440 to 800nm, near-infrared (NIR): from 900 to 1800nm). For the Visible and NIR channels, the two AOTFs will be made of a paratellurite crystal. For the UV channel, the AOTF would have been made from a KDP crystal (potassium dihydrogen phosphate). Unfortunately, the latter did not reach the necessary level of maturity for a space mission, and the KDP-based AOTF was replaced by a stack of Fabry-Pérot interferometers (FPI). Nevertheless, for technological interest, the AOTF approach will also be developed and matured for the UV-channel up to flight level. The focus of this paper is to design for each of the different channels a dedicated RF-chain, containing a RF-generator and a RF-amplifier. The output of the RF-amplifier is tunable to a specific frequency and a specific power level and is injected in the AOTF’s transducer via an impedance matching network. Different architectures are possible to generate the frequency range needed for the different wavelength domains. While the Hilbert Transform solution will be only summarized here and not further investigated, this paper focuses on the Phase Locked Loop (PLL) solution. PLL-solutions are breadboarded for the three wavelength domains. A detailed study is done on the achievable power levels in the Infrared, Visible and the Ultraviolet.@en

Acousto-optical tunable filters (AOTFs) are still little known to the Earth atmosphere remote sensing community. The bulk of passive atmospheric remote sensing instruments remains divided into two families: those relying on interferometric techniques (mostly for the long-wave absorbing species), and those based on diffraction gratings (better suited for UV-VIS absorbing species). Still, AOTFs have some unique features which should deserve more attention, in particular their angular acceptance, and their polarization sensitivity. The first one because it allows to work in an imaging setup, the second because many atmospheric processes have a polarizing effect. In this paper, we will present different AOTF-based instrument concepts (or even prototypes) which take advantage of these features in order to improve the sate-of-the-art of measurement techniques in several fields of atmospheric science. We will first present the improved NO2 camera: its new capabilites, the subsystems which have been changed, and some preliminary results. Then, we will discuss two other potential applications: the study of the solar spectral irradiance variability in the UV, and the detection of auroral polarized emissions. For each concept, we will discuss the current challenges faced by the existing instruments, and analyze how the use of AOTFs could overcome them. A suggestion for the AOTF selection will be made, and the expected instrument performance will be estimated.@en

The abundance of NO2 in the boundary layer relates to air quality and pollution source monitoring. Observing the spatiotemporal distribution of NO2 above well-delimited (flue gas stacks, volcanoes, ships) or more extended sources (cities) allows for applications such as monitoring emission fluxes or studying the plume dynamic chemistry and its transport. So far, most attempts to map the NO2 field from the ground have been made with visible-light scanning grating spectrometers. Benefiting from a high retrieval accuracy, they only achieve a relatively low spatiotemporal resolution that hampers the detection of dynamic features. We present a new type of passive remote sensing instrument aiming at the measurement of the 2-D distributions of NO2 slant column densities (SCDs) with a high spatiotemporal resolution. The measurement principle has strong similarities with the popular filter-based SO2 camera as it relies on spectral images taken at wavelengths where the molecule absorption cross section is different. Contrary to the SO2 camera, the spectral selection is performed by an acousto-optical tunable filter (AOTF) capable of resolving the target molecule's spectral features. The NO2 camera capabilities are demonstrated by imaging the NO2 abundance in the plume of a coal-fired power plant. During this experiment, the 2-D distribution of the NO2 SCD was retrieved with a temporal resolution of 3 min and a spatial sampling of 50 cm (over a 250 × 250m2 area). The detection limit was close to 5 × 1016 molecules cm-2, with a maximum detected SCD of 4 × 1017molecules cm-2. Illustrating the added value of the NO2 camera measurements, the data reveal the dynamics of the NO to NO2 conversion in the early plume with an unprecedent resolution: from its release in the air, and for 100m upwards, the observed NO2 plume concentration increased at a rate of 0.75-1.25g s-1. In joint campaigns with SO2 cameras, the NO2 camera could also help in removing the bias introduced by the NO2 interference with the SO2 spectrum.@en